early paleozoic crust–mantle interaction and lithosphere ...€¦ · early paleozoic granites...
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Lithos 184–187 (2014) 416–435
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Early Paleozoic crust–mantle interaction and lithosphere delamination inSouth China Block: Evidence from geochronology, geochemistry,and Sr–Nd–Hf isotopes of granites
Yan Xia a, Xisheng Xu a,⁎, Haibo Zou b, Lei Liu a
a State Key Laboratory for Mineral Deposits Research, School of Earth Sciences, Nanjing University, Nanjing 210023, Chinab Department of Geology and Geography, Auburn University, Auburn, AL 36849, USA
⁎ Corresponding author. Tel.: +86 25 89680886; fax: +E-mail addresses: [email protected] (Y. Xia), xsx
[email protected] (H. Zou), [email protected]
0024-4937/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.lithos.2013.11.014
a b s t r a c t
a r t i c l e i n f oArticle history:Received 24 July 2013Accepted 23 November 2013Available online 1 December 2013
Keywords:South ChinaEarly Paleozoic granitesZircon U–Pb–Hf isotopesAssimilation fractional crystallization (AFC)processDelamination
The early Paleozoic orogen in South China Block is an intracontinental orogen, and synchronous magmatism(440–390 Ma) is mainly acidic with minor intermediate-mafic magmatism. Previous studies suggest that mostof the early Paleozoic granites in South China belong to peraluminous S-type genesis while amphibole-bearingI-type granites are subordinate. However, our results indicate that considerable amounts of these early Paleozoicgranites have characteristics of both S- and I-type granites. Thus, we propose to divide these granites into twogroups: fewer of them are Group A with relatively high εHf(t) values (clustering within −3.0 to +9.0) andεNd(t) values (−5.2 to +1.3) as well as higher initial temperatures at 810–850 °C, and most of them areGroup B with relatively low εHf(t) values (clustering within−16.0 to −1.0) and εNd(t) values (−13.2 to −4.1)as well as relatively low initial temperatures at 700–830 °C. The Xiawanmonzogranite and Duntou granodioriteare typical Group A granitoids and yield zircon U–Pb ages of ca. 410 Ma. These two granites are characterized byhigh SiO2 (between 67.59 and 74.87 wt.%), metaluminous to peraluminous (A/CNK = 0.96–1.48) compositions,and a negative correlation between P2O5 and SiO2. Their biotites belong to magnesium biotites, indicating thatthey have partial features of either I- or S-type granites. Duntou granodiorites exhibit higher εHf(t) values(clustering within +1 to +8) and εNd(t) values (−3.0 to +1.1) while Xiawan monzogranites show relativelylow εHf(t) values (clustering within −1 to +5) and εNd(t) values (−5.0 to −3.7). Group B granitoids arerepresented by the Miao'ershan–Yuechengling batholith, which are characterized by high SiO2 (between 64.57and 77.37 wt.%), metaluminous compositions (A/CNK = 0.90–1.24), and a negative correlation between P2O5
and SiO2. Yuechengling porphyritic amphibole-bearing biotite granites in this batholith contain abundantamphibole, indicating that they are I-type granites. Miao'ershan–Yuechengling batholith also exhibits relativelylow εHf(t) values (−12.7 to −1.8) and εNd(t) values (−8.9 to−6.7).Geochemical and isotopic analyses on early Paleozoic granites, mafic enclaves and mafic to intermediate rocksdemonstrate that the Group A granitoids including Xiawan and Duntou granites may be generated by AFCprocesses with interactions between asthenosphere-derived magma and metasedimentary rocks, and theGroup B granitoids may be formed by AFC processes with interactions between synchronous basaltic magmaandmetamorphic basement. The post-collisional delamination and asthenospheric upwelling directly participatein the generation of Group A granitoids but indirectly induce the formation of Group B granitoids.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
The South China is tectonically divided into two major blocks:the Yangtze Block to the northwest and the Cathaysia Block tothe southeast, which amalgamated during Neoproterozoic time(e.g., Charvet et al., 1996; Guo et al., 1985, 1989; Li, 1998; Li et al.,1995, 2002, 2008, 2009; Wang et al., 2006, 2007b, 2013a; Zhao andCawood, 1999; Zhou et al., 2009). The boundary between the Yangtze
86 25 [email protected] (X. Xu),(L. Liu).
ghts reserved.
Block and Cathaysia Block is the northeasterly trending Pingxiang–Jiangshan–Shaoxing Fault, but the southwestern extension of thisboundary is unclear because of poor exposure of outcrops and multipleintensive tectonic modifications. Either the Anhua–Luocheng Fault orthe Chenzhou–Linwu Fault has been suggested as the SW boundary bydifferent researchers (Fig. 1a, e.g., Chen and Jahn, 1998; Wang et al.,2008; Zhang et al., 2012; Zhao et al., 2013b). The South Chinawas subse-quently overprinted by at least three tectonothermal events in the earlyPaleozoic (Caledonian), Triassic (Indosinian) and Jurassic–Cretaceous(Yanshanian) (e.g., Charvet, 2013; Charvet et al., 1999, 2010; Faureet al., 2009; Li, 1998; Li and Li, 2007; Li et al., 2010c; Ren, 1991; Wanget al., 2005a,2005b, 2011a, 2012a, 2013a; Xu et al., 2007).
32o
122o114o 116o 118o 120o
121o119o117o115o113o111o
24o
26o
28o
22o
109o
Wuhan
Shanghai
Hangzhou
Nanchang
Changsha
Fuzhou
Taipei
Hongkong
Guangzhou
Qinling-Dabie
Zhejiang
Fujian
Jiangxi
Hunan
GuangdongGuangxi
Xiawan monzogranite
Duntou granodiorite
Granite porphyry
Quartz monzonite
Quartz diorite porphyry
Quartz porphyry
Precambrian basement
Quaternary sediment
Fault
Unconformity boundary
XW01XW02
XW04
XW06
XW07
DT01
DT03DT04
DT06
DT07DT08DT09
DT02
Longquan
b
Early Paleozoic granites (Group A) Early Paleozoic granites (Group B) Early Paleozoic basaltic rocks
Known extent of early Paleozoic orogen in South China Metamorphic core of the orogen
Wuyi Domain
Yunkai Domain
Nanling Domain
Xiawan
Duntou
ShaxiWeipu
Xinyi
A B
C
-gnaixgniP
tluaFgnixoahSYangtze
Cathaysia
Jiangshan-
Anhua-Luoch
eng
Fault
Ch
enzh
ou
-Lin
wu
Fau
lt
110°E
30°N
25°N
105°E
20°N
115°E 120°E
North ChinaBlock
South ChinaBlock
32o
a b
Fig. 1. (a) Schematic map showing the distribution of early Paleozoic igneous rocks in South China (modified after Li et al., 2010c; Sun, 2006; Wang et al., 2013b; Yao et al., 2012).(b) Geological map of Xiawan and Duntou granites. Sample locations are also indicated.
417Y. Xia et al. / Lithos 184–187 (2014) 416–435
Contemporaneous with considerable granitic intrusions and coevalvolcanism, the Yanshanian event has been systematically studied andinterpreted as an extensional tectonics with large-scale mantle–crustinteraction in response to the subduction of the Paleo-Pacific Plate(He and Xu, 2012; He et al., 2010a; Li and Li, 2007; Liu et al., 2012; Xieet al., 2006; Zhou et al., 2006). Owing to the lack of any synchronousintermediate-mafic volcanics, recent studies suggest that boththe early Paleozoic and early Mesozoic (Indosinian) orogens are intra-continental (Charvet et al., 2010; Chu et al., 2012; Faure et al., 2009;Li, 1998; Li et al., 2010c; Yang et al., 2010; Wang et al., 2005b, 2012a).The widespread early Paleozoic and early Mesozoic granites in SouthChina have been regarded as predominantly peraluminous, purecrust-derived S-type granites (Charvet, 2013; Li et al., 1989; Wanget al., 2011a, 2013a; Zhang et al., 2012; Zhou et al., 2006). However,the reports of Indosinian alkaline syenites and A-type granites indicatean extensional tectonic environment for South China during Triassicwith possible basaltic magmatic underplating (Sun et al., 2011; Wanget al., 2005a; Xia et al., 2012; Zhao et al., 2013a).
Recently, Yao et al. (2012) and Wang et al. (2013b) reportedthe presence of high-Mg basalts, andesites, dacites (ca. 435 Ma) andgabbros (466–420 Ma) thatmay be related to lithosphere delamination,indicating the early Paleozoic mantle–crust interaction in South China(Fig. 1a). The early Paleozoic granites in South China Block can bedivided into two groups: Group A in the center of metamorphiccore of the orogen with relatively high εHf(t) value (−9.0 to +10.3,clustering within−3.0 to+9.0) and εNd(t) (−5.2 to +1.3), suggestingthe significant contribution of juvenile mantle-derived materials,and Group B widespreaded in orogen with relatively low εHf(t) value(−35.4 to +2.4, clustering within −16.0 to −1.0) and εNd(t) (−13.2to −4.1), suggesting an ancient recycled crustal source (see inAppendix Table 1 and related references in it). In addition, some of
the Group B granitoids also contain mafic enclaves, which suggeststhat even some Group B granites with low radiogenic Hf and Ndisotopes are not pure crust-derived but crust–mantle mixed source(e.g., Banshanpu, Hongxiaqiao, Daning, Shanzhuang, Hongjiang andXuehuading; Cheng et al., 2009a; Lou et al., 2005; Sha and Yuan, 1991;Wu and Zhang, 2003; Xu et al., 2006; Zhang et al., 2012). In comparisonwith the intensively studied Group B granitoids, systematic studies ofGroup A granitoids lag far behind and consequently our understandingof the early Paleozoic magmatism in South China is at present highlyincomplete. We thus conducted a detailed geochronological andpetrological study of two early Paleozoic Group A granites in SouthChina Block in order to better understand the tectonic environmentof this early Paleozoic tectonothermal event and contemporarymagmatism.
2. Geologic background and samples
The early Paleozoic orogenic belt spans the southeastern half of theSouth China Block (Fig. 1a), stretching for ~2000 km in a northeasterlydirection (Ren et al., 1997), distributing from the Korean Peninsula(Charvet et al., 1999; Kim et al., 2006) to the Indochina block(Ren, 1991; Roger et al., 2007). The orogen was originally assigned tothe Kwangsian orogeny (Ting, 1929), and was subsequently namedSouth China Caledonian Fold Belt (Huang, 1978), Cathaysian or HuananCaledonides (Chang, 1996; Charvet et al., 1999; Ren, 1991), Wuyishan–Yunkaidashan belt (Zhang et al., 1984), Wuyi–Yunkai tectonic zone(Zhang et al., 1991), early Paleozoic Orogen of the South China Block(Faure et al., 2009). Recently, Li et al. (2010c) interpreted it asthe Wuyi–Yunkai orogeny. Wang et al. (2012a) used the previousKwangsian orogeny to define the early Paleozoic event in South China.
418 Y. Xia et al. / Lithos 184–187 (2014) 416–435
With visible angular unconformity betweenpre-Devoniandeformedrocks and Devonian strata (Grabau, 1924; Huang et al., 1980; Ren,1964, 1991; Ting, 1929; Zhao et al., 1996), this orogen includesNeoproterozoic to Lower Paleozoic non-metamorphic or low-grademetamorphic sediments, Paleoproterozoic to Lower Neoproterozoic(Li, 1997; Li et al., 2002) high-grade metamorphic rocks, migmatitesand granitoids. The high-grade metamorphic rocks which reachamphibolite to granulite facies occur mainly in the northeasternsegment, including northern Fujian and southern Zhejiang provinces,and in the Nanling Domain (Fig. 1a) (e.g. FBGMR, 1985; Gan et al.,1993, 1995; Hu et al., 1991; Li, 1988, 1989; Li et al., 1993; Shui et al.,1988; Yu et al., 2006; ZBGMR, 1989; Zhao and Cawood, 1999). In thenorthern Wuyi Domain (Fig. 1a), amphibolite facies metamorphismoccurred between ca. 460 and 445 Ma, whereas cooling below500–300 °C took place by ca. 420 Ma (Li et al., 2010c). The orogenexhibits a near-isothermal decompression clockwise pressure–temperature (P–T) path and a maximum pressure of N0.8 GPa, whichis revealed by the sequence of mineral assemblages and metamorphicreactions for schist, amphibolite and granulites (Li et al., 2010c; Yuet al., 2003a, 2003b, 2005b, 2006, 2007; Zhao and Cawood, 1999),indicating crustal thickening during the orogeny. In the southernWuyi Domain (Fig. 1a), Yu et al. (2003a; 2005a, 2005b) reportedthe occurrence of the Taoxi pelitic granulites with the metamorphicage of 445–479 Ma, a peak metamorphic pressure of 1.0–1.1 GPa andtemperatures of 835–878 °C. In the Yunkai Domain (Fig. 1a), garnet-bearing amphibolites occur as interlayers, lens and pods within thegneiss and schist in the Yunkai Complex or granulite enclave in thecharnockite and cordierite granite (Chen and Zhuang, 1994; GBGMR,1988; Peng et al., 2000), reflecting metamorphic temperatures of780–880 °C and pressures of 0.53–0.71 GPa (Chen and Zhuang, 1994;Qin et al., 2006; Yu et al., 2007; Zhou et al., 1994a).
The Xiawan monzogranite and Duntou granodiorite are located atthe southwest of Longquan city in southwestern Zhejiang Province,with an outcrop area of ~3 km2 and ~16 km2, respectively. Bothgranites intrude into the Paleoproterozoic Badu complex. Sampleswere collected from Xiawan and Duntou granites for this study(Fig. 1b). Sampling locations (latitude and longitude) and mineralassemblages of the samples are listed in Table 1. Except that Xiawanmonzogranite contains more K-feldspars than Duntou granodiorite,the two granites have similar mineral assemblages of quartz +K-feldspar + plagioclase + biotite ± muscovite. The plagioclases inXiawan and Duntou granites have relatively high An contents (An15–30
for Xiawan monzogranite and An25–35 for Duntou granodiorite),implying a relatively high melt temperature (Dall'Agnol et al., 1999;Scaillet et al., 1995). It is noted that all muscovites are anhedral,suggesting they may be of secondary origin. Representative sampleswere analyzed for zircon U–Pb dating, Hf-isotope compositions andwhole-rock major and trace element and Sr–Nd isotope compositions.
Table 1Lithologies and mineral assemblages of Xiawan and Duntou granites.
Graniticcomplexes
Main minerals SampleNo.
Location (GPS position)
Xiawanmonzogranite
Quartz +K-feldspar +plagioclase +biotite ± muscovite
XW01 28°02′38.9″N, 119°05′43.7″EXW02 28°02′26.5″N, 119°05′40.3″EXW04 28°02′49.78″N, 119°06′10.10″EXW06 28°02′46.35″N, 119°06′15.13″EXW07 28°02′47.82″N, 119°06′18.75″E
Duntougranodiorite
Plagioclase +quartz +K-feldspar +biotite ± muscovite
DT01 27°58′38.2″N, 119°03′50″EDT02 27°57′56.1″N, 119°03′17.3″EDT03 27°56′36.2″N, 119°02′0.3″EDT04 27°56′30.4″N, 119°02′12.4″EDT06 27°57′12.57″N, 119°02′23.95″EDT07 27°56′7.68″N, 119°02′16.18″EDT09 27°56′27.48″N, 119°03′15.75″E
3. Analytical methods
3.1. Major element compositions of minerals
All elemental analyses of minerals were obtained from polishedthin sections using a JEOL JXA-8800 electron microprobe at State KeyLaboratory for Mineral Deposits Research at Nanjing University, China.Element determinations (Si, Al, FeT, Mg, Ti, Mn, Na, K, F and Cl) ofbiotites were carried out using a beam size of 1 μm, an acceleratingpotential voltage of 15 kV, and a probe current of 20 nA. The standardsused were hornblende (for Si, Ti, Al, Fe, Ca, Mg, Na and K) and fayalite(forMn).Matrix effectswere corrected using the ZAF software providedby JEOL. The accuracy of the reported values for the analyses is 1%–5%depending on the abundance of the element.
3.2. U–Pb dating of zircons
Zircons were extracted using standard density and magneticseparation techniques. Random zircon grains were handpicked undera binocular stereomicroscope and mounted in a 1.4 cm diameterepoxy disk, and polished to expose the central parts of the grains.In order to characterize the internal structures of the zircons and tochoose appropriate target sites for U–Pb and Hf isotope analyses,cathodoluminescence (CL) imaging was done using a Quanta 400FEGenvironmental scanning electron microscope equipped with an Oxfordenergy dispersive spectroscopy system and aGatan CL3+ detector at theState Key Laboratory of Continental Dynamics, Northwest University,Xi'an for most of the samples, and Hitachi S2250-N scanning electronmicroscope at the Beijing SHRIMP Center, Chinese Academy ofGeological Sciences for the rest. The operating conditions for the CLimaging were at 15 kV and 20 nA.
Zircon U–Pb dating were carried out using an Agilent 7500a ICP-MSequipped with a New Wave 213 nm laser sampler in the State KeyLaboratory of Mineral Deposits Research, Nanjing University. Details ofinstrument settings and analytical procedures follow Jackson et al.(2004). Analyses were carried out with a beam diameter of 25 μm,5 Hz repetition rate, and energy of 10–20 J/cm2. Data acquisitionfor each analysis took 100 s (40 s on background and 60 s on signal).The raw ICP-MS data were processed using GLITTER (Van Achterberghet al., 2001). Common Pb was corrected according to the methodproposed by Andersen (2002). The age calculations and plotting ofconcordia diagrams were made using Isoplot (ver. 3.23) (Ludwig,2003).
3.3. Hf-isotope analysis of zircon
In situ Hf isotopic analysis of zircons were conducted using a Nuplasma MC-ICPMS, equipped with a 213 nm laser sampler at theInstitute of Geochemistry, Chinese Academy of Sciences in Guiyang,China. The analysis was done with ablation pit of 60 μm in diameter,repetition rate of 10 Hz, ablation time of 60 s, and laser beam energyof 0.155 mJ/pulse. In order to evaluate the reliability of the data, zirconstandard 91500 was analyzed during the course of this studyand yielded a weighted mean 176Hf/177Hf ratio of 0.282306 ± 24(2σ), which is in good agreement with the solution analysis result(0.282306 ± 8; Woodhead et al., 2004). The analytical details andinterference correction method of 176Yb on 176Hf are given in Tanget al. (2008). The measured 176Lu/177Hf ratios and the 176Lu decayconstant of 1.867 × 10−11 yr−1 (Söderlund et al., 2004) wereused to calculate initial 176Hf/177Hf ratios. The chondritic values of176Lu/177Hf = 0.0336 ± 1 and 176Hf/177Hf = 0.282785 ± 11 (2σ)(Bouvier et al., 2008) were used for calculating εHf values. Thedepleted mantle Hf model ages (TDM) were calculated using themeasured 176Lu/177Hf ratios based on the assumption that thedepleted mantle reservoir has a linear isotopic growth from176Hf/177Hf = 0.279718 at 4.55 Ga to 0.283250 at present, with
Table 2Representative microprobe analyses of biotites in the Xiawan and Duntou granites.
Sample DT01 DT04 DT09 XW07
No. 1 4 7 1 2 3 5 8 9 11 12 13 2 3 4 5 6 1 2 5 6 7 8
SiO2 26.03 26.43 25.56 27.87 25.09 28.77 25.31 26.82 24.98 24.97 25.15 24.38 25.65 25.15 25.20 26.11 26.08 26.68 26.01 25.98 26.18 25.85 25.49TiO2 0.10 0.09 0.09 1.09 0.11 3.49 0.08 0.49 0.14 0.07 0.11 0.08 0.07 0.08 0.31 0.07 0.08 0.03 0.02 0.07 0.03Al2O3 23.18 20.94 19.67 20.61 19.69 18.89 19.27 19.26 20.94 20.49 20.84 21.28 19.87 20.09 20.13 20.86 21.29 19.12 19.31 19.56 19.81 19.15 18.57FeOT 28.25 26.99 26.40 28.64 31.29 24.70 31.86 31.75 33.19 32.66 31.43 31.52 26.57 26.41 25.50 25.17 25.78 28.55 28.43 27.67 26.92 27.89 27.99MnO 0.40 0.37 0.37 0.49 0.52 0.40 0.57 0.52 0.73 0.58 0.54 0.55 0.83 0.82 1.01 0.97 0.81 0.53 0.59 0.53 0.49 0.55 0.55MgO 13.82 15.10 15.37 9.51 10.53 8.88 10.42 11.54 10.57 11.04 11.06 10.06 15.35 15.29 15.25 15.68 15.59 14.93 14.09 14.65 14.92 14.76 13.92CaO 0.01 0.08 0.09 0.91 0.11 3.46 0.21 0.17 0.13 0.05 0.07 0.05 0.06 0.02 0.25 0.08 0.05 0.09 0.08 0.08 0.08 0.05 0.01Na2O 0.04 0.00 0.05 0.02 0.02 0.00 0.06 0.00 0.00 0.00 0.03 0.03 0.01 0.07 0.01 0.01K2O 0.02 0.01 0.02 0.94 0.09 1.30 0.04 0.26 0.02 0.02 0.07 0.27 0.00 0.04 0.02 0.02 0.00 0.01 0.00 0.06 0.00 0.01F 0.03H2Oa 3.76 3.69 3.58 3.65 3.48 3.66 3.48 3.63 3.58 3.56 3.56 3.50 3.61 3.58 3.59 3.67 3.69 3.65 3.58 3.60 3.62 3.58 3.50Total 95.62 93.71 91.14 93.77 90.92 93.62 91.25 94.50 94.30 93.45 92.82 91.68 92.01 91.44 91.27 92.65 93.40 93.61 92.12 92.08 92.23 91.87 90.06O = F,Cl 0.00 0.00 0.00 0.01 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Total 95.62 93.71 91.14 93.77 90.92 93.60 91.25 94.50 94.29 93.45 92.82 91.68 92.01 91.43 91.27 92.65 93.40 93.61 92.12 92.08 92.22 91.87 90.06
Cations based on 22 oxygen anionsSi 4.155 4.291 4.278 4.576 4.324 4.689 4.360 4.436 4.180 4.206 4.235 4.173 4.262 4.207 4.210 4.268 4.235 4.383 4.351 4.328 4.335 4.326 4.366AlIV 3.845 3.709 3.722 3.424 3.676 3.311 3.640 3.564 3.820 3.794 3.765 3.827 3.738 3.793 3.790 3.732 3.765 3.617 3.649 3.672 3.665 3.674 3.634AlVI 0.515 0.298 0.158 0.565 0.323 0.318 0.271 0.190 0.310 0.274 0.371 0.465 0.154 0.167 0.175 0.288 0.310 0.085 0.157 0.168 0.202 0.104 0.114Ti 0.012 0.011 0.011 0.135 0.014 0.427 0.011 0.061 0.018 0.009 0.014 0.010 0.008 0.010 0.039 0.008 0.010 0.003 0.000 0.002 0.009 0.004 0.000Fe 3.771 3.665 3.696 3.933 4.510 3.367 4.590 4.391 4.645 4.600 4.426 4.512 3.691 3.694 3.564 3.442 3.502 3.923 3.977 3.854 3.728 3.903 4.008Mn 0.054 0.051 0.053 0.069 0.075 0.056 0.083 0.073 0.103 0.083 0.077 0.079 0.117 0.116 0.143 0.135 0.111 0.074 0.084 0.075 0.068 0.078 0.080Mg 3.289 3.654 3.835 2.328 2.706 2.158 2.675 2.845 2.636 2.772 2.776 2.566 3.801 3.812 3.798 3.821 3.774 3.657 3.512 3.637 3.684 3.682 3.553Ca 0.002 0.014 0.016 0.159 0.021 0.604 0.039 0.030 0.023 0.009 0.013 0.009 0.011 0.004 0.045 0.014 0.009 0.015 0.014 0.014 0.014 0.009 0.002Na 0.012 0.001 0.000 0.014 0.008 0.005 0.001 0.019 0.000 0.000 0.000 0.001 0.000 0.001 0.001 0.008 0.000 0.009 0.002 0.000 0.021 0.002 0.004K 0.004 0.001 0.004 0.197 0.019 0.270 0.009 0.055 0.005 0.005 0.015 0.060 0.000 0.000 0.007 0.005 0.004 0.001 0.002 0.000 0.012 0.001 0.001OHa 3.999 3.997 3.999 3.994 3.996 3.982 3.998 4.000 3.995 3.998 3.999 4.000 3.999 3.999 3.999 3.999 4.000 3.999 3.999 4.000 3.994 3.998 4.000F 0.000 0.000 0.000 0.000 0.000 0.014 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Y total 7.642 7.679 7.753 7.029 7.628 6.327 7.630 7.560 7.711 7.739 7.663 7.632 7.773 7.799 7.719 7.693 7.707 7.743 7.730 7.736 7.692 7.771 7.755X total 0.019 0.016 0.020 0.371 0.048 0.880 0.049 0.104 0.028 0.014 0.027 0.069 0.011 0.005 0.054 0.027 0.013 0.025 0.018 0.014 0.047 0.012 0.007Al total 4.360 4.007 3.880 3.989 3.999 3.629 3.912 3.754 4.129 4.068 4.136 4.293 3.892 3.960 3.965 4.020 4.075 3.703 3.807 3.840 3.867 3.778 3.748
a H2O calculations after Tindle and Webb, 1990.
419Y.X
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7.0 6.5 6.0 5.5 5.0 4.5 4.0Si
0.0
0.2
0.4
0.6
0.8
1.0
FeT
/(F
eT+
Mg)
Biotites
Phlogopites
Eastonite
SiderophyliteAnnite
Phlogopite
Alteration trend
0 5 10 15 20 25 30MgO(wt.%)
Phlogopitemantle-derived rock
Magnesium biotitemixture-derived rock
Ferrum biotitecrust-derived rock
5 10 15 20 25 30 35 40 455
10
15
20
25
30
FeO(wt.%)
P
A
C
5 10 15 200MgO(wt.%)
P
A
C
a b
c d
Duntou pluton
Xiawan pluton
Al 2
O3(
wt.%
)
Fig. 2. (a) FeT/(FeT + Mg) vs. Si diagram for biotite. (b) FeT/(FeT + Mg) vs. MgO diagram for biotite (after Zhou, 1988). (c, d) Al2O3 vs. FeO and MgO diagrams for biotite classification(after Abdel Rahman, 1994). A, biotite in anorogenic alkaline suites; P, biotite in peraluminous granites (S-type granites); C, biotite inmetaluminous calc-alkaline granite suites (I-type granites).
420 Y. Xia et al. / Lithos 184–187 (2014) 416–435
176Lu/177Hf = 0.0384 (Griffin et al., 2000). The new continentalcrust Hf model ages (TNC) were calculated using the measured176Lu/177Hf ratios based on the assumption that the new continen-tal crust reservoir (island arcs) has a linear isotopic growth from176Hf/177Hf = 0.279703 at 4.55 Ga to 0.283145 at present, with176Lu/177Hf = 0.0375 (Dhuime et al., 2011). The TNC provides abetter constraint than TDM on when the continental crust generated(Dhuime et al., 2011). We also present a two-stage model age (T2DMor T2NC) for each zircon, which assumes that its parental magma wasproduced from an average continental crust (176Lu/177Hf = 0.015;Griffin et al., 2002) thatwas originally derived from the depletedmantleor island arcs.
100µm
100µm
Fig. 3. CL images of representative zircons from Xiawan and Duntou granites. Small solid circleanalyses.
3.4. Major and trace element analyses of whole-rocks
Bulk-rock major element analysis was performed using anARL9800XP + X-ray fluorescence spectrometer (XRF) at the Center ofModern Analysis, NJU. The analytical precision is generally better than2% for all elements. Trace element analyses were measured by ICPMS(Agilent 7500a) at Key Laboratory of Continental Dynamics, NorthwestUniversity, Xi'an. For trace element analysis, sample powders weredigested using an HF + HNO3 mixture in high-pressure Teflon bombsat 190 °C for 48 h. Analyses of rock standards BCR-2, BHVO-2 andAGV-2 indicate that precisions of trace element analyses are generallybetter than 5% and accuracies are better than 10%.
100µm
100µm
100µm
100µm
s are spots for U–Pb isotope analyses, and big white dashed circles are spots for Hf isotope
XW-02
402
410
418
426
0.076
0.072
0.068
0.064
0.0600.4 2 0.4 6 0.540.5 0 0.58 0.62 0.66
390
410
430
450
470
n=7436±6MaMSWD=1.5
(2σ)
n=11413±3MaMSWD=0.25
(2σ)
XW-01
396
404
412
420
428
0.073
0.071
0.069
0.067
0.065
0.063
0.061
0.0590.40.3 0.5 0.6 0.7
420
440
400
380
n=19413±3MaMSWD=0.073
(2σ)
DT-01
385
395
405
415
425
0.25
0.059
0.057
0.061
0.063
0.065
0.067
0.069
0.071
0.073
390
370
430
410
450
n=19409±3MaMSWD=0.116
(2σ)
DT-02
380
400
420
440
470
450
430
410
390
370
0.50.3 5 0.4 5 0.5 5 0.6 5 0.7 5 0.1 0.3 0.7 0.90.056
0.060
0.064
0.068
0.072
0.076
n=17411±4MaMSWD=0.48
(2σ)
DT-04
396
404
412
420
0.074
0.070
0.066
0.062
0.058
460
420
440
400
380
n=6436±5MaMSWD=0.42
(2σ)
n=14410±3MaMSWD=0.33
(2σ)
.0.069
0.067
0.065
0.063
0.0610.3 5 0.4 5 0.5 5 0.6 5 0.7 5 0.4 4 0.4 8 0.5 2 0.560.5 40.5 00.4 6
420
410
400
390
418
414
410
406
402
398
DT-09n=7411±4MaMSWD=0.17
(2σ)
206 P
b/23
8 U20
6 Pb/
238 U
206 P
b/23
8 U
207Pb/235U 207Pb/235U
Fig. 4. U–Pb Concordia diagram of representative zircons from Xiawan and Duntou granites.
421Y. Xia et al. / Lithos 184–187 (2014) 416–435
3.5. Sr–Nd isotope analysis of whole-rocks
All Rb–Sr and Sm–Nd isotope analyses were carried out at the KeyLaboratory of Crust–Mantle Materials and Environments, University ofScience and Technology of China. Following sample dissolution, Rb,Sr and the light rare-earth elements were isolated on quartz columnsby conventional-ion exchange chromatography with a 5-ml resin bedof AG 50 W-X12 (200–400 mesh). Nd and Sm were separated fromother rare earth elements on quartz columns using 1.7 ml Teflonpowder coated with HDEHP, di(2-ethylhexyl) orthophosphoric acid,as cation exchange medium. Sr was loaded with a Ta-HF activatoron pre-conditioned Ta filaments. Nd was loaded as phosphate onpreconditioned Re filaments. Sr and Nd isotopic data were obtainedusing a FinniganMAT-262 mass spectrometer. Sr and Nd isotopic ratioswere corrected for mass fractionation relative to 86Sr/88Sr = 0.1194
and 146Nd/144Nd = 0.7219, respectively. Analyses on the standardsolutions of NBS 987 and La Jolla yielded mean values of 0.710249 ±0.000012 (2σ, n = 38) for the 87Sr/86Sr ratio and 0.511869 ±0.000006 (2σ, n = 25) for the 143Nd/144Nd ratio, during the course ofthis study. More details on analytical procedures are given in Chenet al. (2000) and Chen et al. (2007). For the calculation of (87Sr/86Sr)i,εNd(t) and Nd model ages, the following parameter were used:λRb = 1.42 × 10−11 year−1 (Minster et al., 1982); λSm =6.54 ×10−12 year−1 (Lugmair and Marti, 1978); (147Sm/144Nd)CHUR =0.1960 ± 4, (143Nd/144Nd)CHUR = 0.512630 ± 11 (2σ) (Bouvier et al.,2008); (143Nd/144Nd)DM = 0.513151, (147Sm/144Nd)DM = 0.2136(Liew and Hofmann, 1988). The 147Sm/144Nd value of 0.118 for av-erage continental crust (Jahn and Condie, 1995) was used for themantle extraction model age (T2DM) for the source rocks of themagmas.
XW01XW02DT01
DT04DT09
DT02
400 410 420 430 440 450 4603090
0.5
1.0
1.5
2.0
2.5
3.0
Age(Ma)
Th/U=0.4
Th/U
Fig. 5. Th/U vs Age diagram for zircons from Xiawan and Duntou granites.
422 Y. Xia et al. / Lithos 184–187 (2014) 416–435
4. Analytical results
4.1. Biotite compositions
The compositions of biotites in Xiawan monzogranite and Duntougranodiorite are shown in Table 2. Because of chloritization, the totalmajor element contents are low without analyzing H2O. Watson andHarrison (1983) suggested that the chemical changes for the trans-formation from biotite to chlorite were as follows: large introductionsof H2O and H+; relatively minor introductions of Mg and Fe; andmajor losses of K and Si. Since themajor element composition of graniticchlorite is strongly influenced by the composition of the parent biotite(Dodge, 1973), the chlorites have similar FeOT/(FeOT + MgO) ratiosto those of their host biotites (Tulloch, 1979). According to the revisedmica classification (Rieder et al., 1998), all of the chloritic biotites inXiawan and Duntou granites plot in between the Fe-rich siderophylliteend member and Mg-rich eastonite end member (Fig. 2a), and mostof these chloritic biotites belong to magnesium biotites (Fig. 2b).
4.2. Zircons U–Pb geochronology
Two samples (XW01 and XW02) from Xiawan monzogranite andfour samples (DT01, DT02, DT04 and DT09) from Duntou granodioriteare selected for zircon U–Pb dating and Hf isotope analyses. Thelocations of these 6 selected samples are shown in Fig. 1b. CL imagesof representative zircons are shown in Fig. 3. Zircons separated from
1.0GaDM
2150010005000
0
-10
-20
-40
10
20
-30
ε Hf(t
)
17 6
17 7
Lu / Hf=0.01 5
NC
CHUR
Age
Duntou gXiawan monzogranite
Fig. 6. Hf-isotope compositions of representative zircons from Xiawan and Duntou granites. Dreferences: Xu et al., 2005; Yu et al., 2005; Zeng et al., 2008; He et al., 2010b; Liu et al., 2010; Li etet al., 2012; Zhang et al., 2010a, 2011a, 2011b, 2012; Wang et al., 2011a, 2013b; Zhao et al., 20
the selected samples are light yellow, prismatic or ellipsoidal in shapewith aspect ratios of 1.5 to 4.0, and approximately 100 to 250 μm longwith well-developed oscillatory zoning. Some euhedral zircons showprismswith {110} N {100} and pyramidswith {211} N {101}, and othersare ellipsoidal shape.
4.2.1. Xiawan monzogranite (XW01 and XW02)Nineteen U–Pb analyses of zircons from XW01 show variably high
Th/U (0.11–2.09), indicating an igneous origin (Fig. 5). All the analysesare concordant, yielding a weighted mean 206Pb/238U age of 413 ±3 Ma (MSWD = 0.073; Appendix Table 2, Fig. 4).
Zircons from the XW02 have high and varying Th/U values (0.96 to2.49) (Fig. 5). Of the 18 analyses, seven grains record older 206Pb/238Uages of 436 ± 6 Ma (MSWD = 1.5; Appendix Table 2, Fig. 4). Theremaining 11 analyses plot on the concordia curve and yield a weightedmean 206Pb/238U age of 413 ± 3 Ma (MSWD = 0.25; AppendixTable 2, Fig. 4).
4.2.2. Duntou granodiorite (DT01, DT02, DT04 and DT09)A total of 19 analyses on 19 zircons fromDT01 give high Th/U values
(0.12 to 2.05) (Fig. 5). These analyses plot on or close to the concordiacurve, yielding 206Pb/238U ages of 406 ± 5 Ma to 413 ± 6 Ma witha weighted mean 206Pb/238U age of 409 ± 3 Ma (MSWD = 0.116;Appendix Table 2, Fig. 4).
Zircons from DT02 show high Th/U values (0.14 to 2.72) and areconsistent with their magmatic origin (Fig. 5). Eighteen analyses exceptfor DT02-16 give 206Pb/238U age of 453 ± 9 Ma, and other seventeenanalyses yield 206Pb/238U ages primarily between 397 ± 11 and429 ± 10 Ma with a weighted mean 206Pb/238U age of 411 ± 4 Ma(MSWD = 0.48; Appendix Table 2, Fig. 4).
Twenty analyses of 20 zircons from DT04 were obtained, showinghigh Th/U values (0.99–1.87) (Fig. 5). All the analyses are concordant.Six zircon grains exhibit older 206Pb/238U ages ranging from 431 ±6 Ma to 443 ± 7 Ma and give a weighted mean age of 436 ± 5 Ma(MSWD = 0.42; Appendix Table 2, Fig. 4). The other 14 zircon grainsexhibit younger 206Pb/238U ages ranging from 403 ± 7 Ma to 415 ±5 Ma with a weighted mean age of 410 ± 3 Ma (MSWD = 0.33;Appendix Table 2, Fig. 4).
Seven analyses on seven grains from DT09 give high and varyingTh/U values (0.05–0.84) (Fig. 5). The bulk of the analyses are groupedand concordant, defining a weighted mean 206Pb/238U age of 411 ±4 Ma (MSWD = 0.17; Appendix Table 2, Fig. 4).
2.5Ga
4000350030002500000
3.8Ga
(Ma)
-40
-30
-20
-10
0
10
20
400 420 440 460 480 500 520
CHUR
NC
DM
Metamorphic rocks
Basic–intermediate rocks and mafic enclaves
GroupBOther of groupAranodiorite
ata sources are the same as in Appendix Table 1. Data of Hf-isotope are from the followingal., 2010a, 2010b, 2011; Chu et al., 2012;Guo et al., 2012; Nong et al., 2012; Yang, 2012; Yao13b and this study.
Table 3Chemical compositions of representative samples from Xiawan and Duntou granites.
Sample Xiawan monzogranite Duntou granodiorite
XW01 XW04 XW06 XW07 DT01 DT02 DT03 DT04 DT06 DT07 DT09
SiO2 73.19 72.97 72.44 72.25 67.59 68.89 74.36 73.51 74.87 72.95 74.63TiO2 0.28 0.30 0.32 0.31 0.54 0.49 0.16 0.21 0.08 0.22 0.21Al2O3 13.43 13.47 13.17 13.58 15.69 16.23 13.54 13.85 12.59 13.94 13.26Fe2O3
T 2.04 2.22 2.32 2.15 3.51 3.23 1.28 1.95 0.60 1.86 1.80MnO 0.03 0.02 0.03 0.04 0.05 0.04 0.03 0.03 0.04 0.04 0.05MgO 0.39 0.41 0.54 0.77 1.52 1.15 0.24 0.54 0.13 0.40 0.72CaO 0.61 0.19 0.82 0.97 1.09 0.45 0.16 1.00 1.62 1.03 1.34Na2O 3.30 3.31 3.16 3.89 5.24 4.79 3.57 3.46 3.56 3.50 4.59K2O 4.82 5.27 5.30 4.02 2.01 2.08 5.28 3.65 3.99 3.55 1.51P2O5 0.09 0.05 0.11 0.11 0.23 0.22 0.04 0.08 0.03 0.07 0.04LOI 1.52 1.47 1.45 1.68 2.63 2.60 1.17 1.40 2.33 2.14 1.64SUM 99.69 99.69 99.66 99.77 100.11 100.15 99.82 99.68 99.84 99.71 99.80ALK 8.12 8.59 8.46 7.91 7.25 6.87 8.85 7.12 7.55 7.05 6.10A/CNKa 1.14 1.17 1.06 1.09 1.23 1.48 1.14 1.21 0.96 1.22 1.14K2O/Na2O 1.46 1.59 1.68 1.03 0.38 0.43 1.48 1.06 1.12 1.02 0.33Li 10.3 10.3 25.6 23.5 11.2 20.0 164.5 10.2Be 1.01 1.08 1.06 1.60 2.50 1.29 1.44 1.25Sc 3.16 3.45 3.77 3.40 3.53 2.64 2.77 2.23V 19.2 20.6 24.5 58.1 12.4 18.1 16.0 21.7Cr 4.70 8.47 6.60 14.1 2.13 6.50 2.75 6.00Co 2.57 1.61 2.82 8.12 1.46 2.69 2.30 2.87Ni 2.82 3.05 3.67 8.23 1.02 2.95 1.62 3.21Cu 13.77 5.98 4.62 8.52 2.87 3.16 2.09 3.34Zn 26.9 45.7 45.7 107 47.6 34.9 37.2 46.2Ga 14.3 13.8 13.6 16.7 16.6 13.3 14.9 12.2Ge 0.78 0.71 0.58 0.62 1.34 0.91 0.77 0.84Rb 130 125 147 101 202 91.2 107 72.7Sr 151 172 228 214 137 327 151 225Y 7.79 7.79 7.74 8.31 24.4 7.55 9.35 5.22Zr 245 276 220 219 272 176 182 106Nb 9.61 10.6 9.66 8.07 19.3 4.59 11.5 3.48Cs 1.89 1.87 3.59 2.58 2.84 2.90 3.11 1.77Ba 1568 1888 1249 493 881 2168 1079 262La 64.5 49.3 39.7 36.1 61.3 38.7 49.0 11.6Ce 106 88.0 66.1 60.9 117 63.6 83.5 15Pr 11.1 10.1 6.39 6.38 13.5 6.22 8.01 1.98Nd 35.2 32.3 20.3 21.0 47.7 20.0 25.5 6.85Sm 4.86 4.49 2.95 2.95 8.18 2.98 3.79 1.17Eu 1.08 1.09 0.90 0.85 1.04 1.06 0.94 0.49Gd 3.33 3.02 2.18 2.15 6.61 2.19 2.88 1.04Tb 0.40 0.36 0.27 0.27 0.87 0.27 0.37 0.14Dy 1.83 1.75 1.43 1.41 4.79 1.40 1.80 0.82Ho 0.29 0.29 0.26 0.25 0.87 0.24 0.31 0.15Er 0.77 0.81 0.72 0.73 2.45 0.70 0.83 0.44Tm 0.10 0.11 0.10 0.10 0.35 0.10 0.11 0.07Yb 0.64 0.69 0.65 0.69 2.16 0.71 0.69 0.45Lu 0.10 0.11 0.11 0.10 0.32 0.11 0.11 0.07Hf 5.81 6.42 4.77 4.40 6.88 4.24 4.39 2.82Ta 0.36 0.29 0.34 0.68 1.04 0.38 0.74 0.15Pb 21.9 23.2 21.2 36.1 37.0 24.8 22.8 6.5Th 20.2 19.6 17.3 7.4 23.2 15.7 15.3 3.7U 1.85 0.67 1.18 0.74 3.12 1.27 1.17 0.67Eu/Eu*b 0.78 0.86 1.04 0.98 0.42 1.21 0.84 1.33∑REE 230 192 142 134 267 138 178 41(La/Yb)N 68.3 48.0 41.0 35.5 19.1 36.9 47.7 17.6T (°C)c 836 850 819 824 865 811 815 763
a A/CNK = molar Al2O3/(CaO + Na2O + K2O).b Eu/Eu* = 2 × EuN/(SmN + GdN).c Temperature (°C) is calculated after Watson and Harrison (1983).
423Y. Xia et al. / Lithos 184–187 (2014) 416–435
4.3. Zircons Hf-isotopes
The zircon Hf analyses were measured on the same grains usedfor U–Pb dating (Fig. 3). Analytical results of the Lu–Hf isotopiccompositions are given in Appendix Table 3 and illustrated in Fig. 6.
4.3.1. Xiawan monzogranite (XW01 and XW02)Zircons from XW01 and XW02 show similar Hf-isotope
compositions. The εHf(t) and initial 176Hf/177Hf ratios for zircons inthese two samples are relatively heterogeneous. The εHf(t) values varyfrom −9.0 to +7.2, clustering within the range of −1 to +5 (Fig. 5).
In addition, the initial 176Hf/177Hf ratios range from 0.282272 to0.282730, corresponding to the two-stage depleted mantle Hf modelages (T2DM) of 0.92 Ga to 1.95 Ga or the two-stage new continentalcrust Hf model ages (T2NC) of 0.71 Ga to 1.78 Ga (Fig. 5).
4.3.2. Duntou granodiorite (DT01, DT02, DT04 and DT09)The zircons from DT01, DT02, DT04 and DT09 also exhibit similar
εHf(t) values and initial 176Hf/177Hf ratios. The εHf(t) values vary from−3.9 to +10.3, clustering within the range of +1 to +8 (Fig. 5).In addition, the initial 176Hf/177Hf ratios range from 0.282417 to0.282814, corresponding to the two-stage depleted mantle Hf model
424 Y. Xia et al. / Lithos 184–187 (2014) 416–435
ages (T2DM) of 0.73 Ga to 1.63 Ga or the two-stage new continentalcrust Hf model ages (T2NC) of 0.51 Ga to 1.44 Ga (Fig. 5).
4.4. Whole-rock major and trace elements
A complete data set of whole-rockmajor and trace element analysesfor representative samples from the Xiawan monzogranite and Duntougranodiorite are listed in Table 3. All the samples of Xiawan and Duntougranites show high SiO2 (between 67.59 and 74.87 wt.%) and relativelyhigh total alkalis (K2O + Na2O) ranging from 6.10 to 8.85 wt.%, plottingin the granodiorite and granite fields on the total alkali-silica (TAS)diagram (Fig. 7a). Based on the molar ratios of Al2O3/(CaO + Na2O +K2O) (A/CNK) and Al2O3/(Na2O + K2O) (A/NK) (Fig. 7b), the Xiawanmonzogranite is weakly to strongly peraluminous (A/CNK =1.06–1.17), plotting in the field of post-orogenic granitoids. The Duntougranodiorite is metaluminous to peraluminous (A/CNK = 0.96–1.48),and most of the samples plot in the continental collision granitoidsand post-orogenic granitoids field. In Fig. 7c, all samples of Xiawanand Duntou granites except for DT01, DT02 and DT09 have high K2O(3.55–5.30 wt.%) and very high K2O/Na2O (1.02–1.68), characteristicmainly of crustal origin. The samples of DT01, DT02 and DT09 aredistinct from other samples with its relatively low K2O content(1.51–2.08 wt.%) and high Na2O (4.59–5.24 wt.%), reaching to thefield of Precambrian amphibolites, and show a likely mantle-derivedmagma involvement in Duntou granodiorite. Alternatively, thereduction of K2O content in DT01, DT02 and DT09 may be caused bythe alteration of biotite or other K-rich minerals, especially the LOI ofDT01 and DT02 can reach more than 2.60 wt.% which is the highestof Duntou granodiorite. In K2O–SiO2 (Fig. 8) and K2O–Na2O (Fig. 7c)diagrams, all samples of Xiawan and Duntou granites together with
5040
12
0
16
4
8
SiO2(wt%)80
Alkaline
SubalkalineGranodiorite Granite
Quartzmonzonite
Amphibolites
60 70
Na2O(wt%)
KO
/Na
O=2
2
2
K O/N a O=0. 5
2
2
K2O
(wt%
)
0
2
4
6
8
10
0 1 2 3 4 5 6 7 8
Calc-alkaline
ShoshoniticUltra-potassic
a
c
Na 2
O+
K2O
(wt%
)
Fig. 7. (a) The total alkali vs. silica (TAS) diagram (after Middlemost, 1994) used for the classifiDuntou granites in terms of alumina saturation, the fields of granitoids from different tecton(after Turner et al., 1996). (d) ACF diagram (after White and Chappell, 1977). Symbols areChen et al., 2012; Cheng et al., 2009a, 2009b, 2012; Fu et al., 2004; Geng et al., 2006; He etLou et al., 2005; Peng et al., 2006a, 2006b; Sha and Yuan, 1991; Shen et al., 2008; Wan et al.,2006; Yao et al., 2012; Zeng et al., 2008; Zhang et al., 2010b, 2010c, 2012; Zhao et al., 2013b; Z
other early Paleozoic granites in South China plot in the calc-alkalineto shoshonitic fields. In Harker diagrams (Fig. 8), Xiawan and Duntougranites show negative correlation between P2O5 and SiO2, characteris-tic of I-type granites. The P2O5 and TiO2 saturation thermometer (Greenand Pearson, 1986; Harrison and Watson, 1984) indicates that Xiawanand Duntou granites have high initial temperatures (Fig. 8), whichis in accordance with their Zr saturation temperatures (763–865 °C,most higher than 810 °C; Table 3; Watson and Harrison, 1983).
Xiawan and Duntou granites with total REE ranging from 41 to267 ppm show similar chondrite-normalized REE patterns and relativeenrichment of LREE over HREE without significant negative Eu anoma-lies (Eu/Eu* = 0.42–1.33), suggesting that the magma may be derivedfrom lower crust or mantle-derived materials (Table 3, Fig. 9). The(La/Yb)N values are higher in Xiawan monzogranite (41.0–68.3) thanin Duntou granodiorite (17.6–47.7) (Table 3). In the primitive mantle-normalized trace element diagrams (Fig. 9), they are all characterizedby positive Rb, Ba, Th, U and Pb and negative Nb and Ta anomalies andmarked depletion in Sr, P and Ti. The negative P and Ti anomaliesindicate that fractionation of apatite and Fe–Ti oxides also occurredduring magma evolution. However, all the samples of Xiawan andDuntou together with other early Paleozoic granites in South Chinahave REE patterns and trace element diagrams similar to Precambrianmetamorphic basement rocks, suggesting the possibility that they aremainly derived from the crustal material.
4.5. Sr–Nd isotopes
The whole-rock Nd isotopic data of Xiawan and Duntou granites aregiven in Table 4. Xiawan monzogranites have relatively lower initial87Sr/86Sr ranging from 0.70061 to 0.70419 and higher εNd(t) values
Al2O3/(CaO+Na2O+K2O)molar
AO
3/l 2
(Na 2
O+
K2O
)mol
ar
1.0 1.5 2.00.50.5
1.0
1.5
2.0
2.5
3.0
metaluminous peraluminous
peralkaline
I
S
Grt
Pl
Amp
Ms
Bt
Crd
A=Al-Na-K
C=Ca F=Fe+Mg
CAG
CCG
IAG
OP
POG
CEUGRRG
OP: oceanicplagiogranites
IAG: island arc granitoidsRRG: rift-related granitoids
POG: post-orogenic granitoidsCAG: continental arc granitoids
CCG: continental collision granitoidsCEUG: continental epeirogenic uplift granitoids
b
d
cation of the Xiawan and Duntou samples. (b) Chemical compositions of the Xiawan andic environments are after Maniar and Piccoli (1989). (c) K2O vs. Na2O variation diagramthe same as in Fig. 6. Data of chemical compositions are from the following references:al., 2010b; Li et al., 2006, 2010a, 2010b, 2010c, 2011; Liu et al., 2008; Lou et al., 2002;2010; Wang et al., 2011a, 2011b, 2013b; Wu and Zhang, 2003; Wu et al., 2008; Xu et al.,hou et al., 1994b and this study.
1080 CO900 CO
800 CO
1050 CO
1000 CO
950 CO
900 CO
6050 080704
SiO2(wt%)
0
1
2
3TiO2(wt%)
P2O5(wt%)
0.3
0.2
0.1
0.4
0.5
I-type granites trend
0
2
4
6
8K2O(wt%)
Calc-alkaline
enilaklacissatoP
Tholeiitic
High-K calc-alkaline
Shoshonitic
Amphibolites
Fig. 8. Harker diagram of major-element compositions of the Xiawan and Duntousamples: in K2O vs. SiO2 diagram (after Peccerillo and Taylor, 1976), the dotted linerepresents the division between potassic alkaline and shoshonitic suites (after Calanchiet al., 2002); in P2O5 vs. SiO2 (after Harrison and Watson, 1984), the I-type granitestrend follows Chappell (1999); the TiO2 vs. SiO2 is after Green and Pearson (1986).Symbols and data sources are the same as in Fig. 7.
0.1
1
10
100
1000
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
GroupB
Basic–intermediate rocksand mafic enclaves
Sam
ple/
Cho
ndrit
e
Fig. 9.Chondrite-normalized REEpatterns and Primitivemantle-normalizedmultiple trace elemMcLennan (1985). The primitive mantle values are fromMcDonough and Sun (1995). Symbol
425Y. Xia et al. / Lithos 184–187 (2014) 416–435
ranging from −5.0 to −3.7. The initial 87Sr/86Sr and εNd(t) of Duntougranodiorites vary from 0.70249 to 0.70719 and −3.0 to +1.1respectively.
The Xiawan and Duntou granites and other granitoids of Group Ahave relatively higher radiogenic Nd than Group B granitoids butsome still plot within the evolution domain of Precambrian basementin either Yangtze or Cathaysia (Fig. 10a). They are well distributedalong the terrestrial array of εNd(t) vs εHf(t) diagram (Vervoort et al.,1999), and most of the samples plot into the range of global lowercrust (Fig. 10b).
5. Discussion
5.1. The chronology of early Paleozoic granites in South China
Numerous granitic bodies emplaced during the early Paleozoic tothe east of the Anhua–Luocheng Fault in South China (Fig. 1a), coveringan outcrop area over 20,000 km2 (e.g., Shu et al., 2008a, 2008b; Zhouet al., 2006). Recently SHRIMP and LA-ICP-MS zircon U–Pb datingshow that they were intruded mainly from 381 to 467 Ma (Chen andZhuang, 1994; Chen et al., 2011; Cheng et al., 2009a, 2009b, 2012; Chuet al., 2012; Fu et al., 2004; Guo et al., 2012; He et al., 2010b; Li, 1991;Li et al., 1989, 2010a, 2010b, 2010c; Lou et al., 2005; Nong et al., 2012;Peng et al., 2000, 2006b; Shen et al., 2008; Tang and Li, 2003; Wanet al., 2010; Wang et al., 2011a,2011b; Wu and Zhang, 2003; Wu et al.,2008; Xu and Zhang, 1993; Xu et al., 2006; Zeng et al., 2008; Zhanget al., 2010a,2010b,2010c, 2011b, 2012; Zhao et al., 2013b and thisstudy; also see in Appendix Table 1), mostly between 440 and390 Ma, systematically postdating the peak metamorphism (earlierthan 445 Ma; Appendix Table 1; Yu et al., 2005b, 2006, 2007; Li et al.,2010c), implying that they are not form in the syn-collision compres-sional environment. In addition, Charvet et al. (2010) reported ages of433 ± 9 Ma and 437 ± 5 Ma (Early Silurian) for the main anatecticevent, and a younger age of 412 ± 5 Ma (Late Silurian–Early Devonian)for a late re-heating process by U–Th–Pb EPMA monazite dating.England and Thompson (1984) and Thompson and Connolly (1995)have argued that tens of millions of years may pass between a crustalthickening event and the subsequent melting/emplacement ofsyn-collisional granites. In fact, most of those granites are intrudedinto Paleoproterozoic to Neoproterozoic metamorphic basement(e.g., Mayuan, Badu, Zhoutan and Yunkai Complexes; see in AppendixTable 1), and a substantial proportion of granites develop gneissicstructure with a strong magmatic orientation or a post-magmaticductile deformational texture parallel to the schistosity of metamorphicwall rocks (Wang et al., 2011a, 2013a; Zhang et al., 2012). Thus, all theseavailable age data imply that early Paleozoic granites should belong tosyn-collisional to post-collisional granites.
0.1
1
10
100
1000
RbBaTh U NbTa K LaCePbSr P NdSmZr Hf Eu Ti Tb Y YbLu
10000
Amphibolites
Basementmetasedimentary rocks
Sam
ple/
prim
itive
-man
tle
ent diagrams of theXiawan andDuntou samples. The chondrite values are fromTaylor ands and data sources are the same as in Fig. 7.
Table 4Sr–Nd isotope compositions of representative samples from Xiawan and Duntou granites.
Sample Age (Ma) Rb (ppm) Sr (ppm) 87Rb/86Sr 87Sr/86Sr 2σ 87Sr/86Sri Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd 2σ εNd(t) T2DM (Ga)
Duntou granodioriteDT01 410 102 190 1.5526 0.711557 0.000016 0.70249 3.36 22.9 0.0887 0.5124 0.000015 1.1 1.08DT02 410 97.5 238 1.1868 0.713814 0.000017 0.70688 2.69 16.9 0.0964 0.5124 0.000009 0.2 1.15DT04 410 87.0 278 0.9055 0.712438 0.000017 0.70715 3.21 20.7 0.0938 0.5123 0.000006 −1.5 1.29DT09 410 72.4 195 1.0751 0.711977 0.000017 0.70570 1.32 7.20 0.1107 0.5122 0.000013 −3.0 1.41
Xiawan monzograniteXW01 410 119 132 2.6243 0.715930 0.000016 0.70061 5.54 38.1 0.0879 0.5121 0.000015 −3.7 1.47XW04 410 125 172 2.0962 0.715619 0.000013 0.70338 4.49 32.3 0.0839 0.5121 0.000010 −5.0 1.58XW07 410 147 228 1.8587 0.715045 0.000012 0.70419 2.95 20.3 0.0878 0.5121 0.000005 −4.2 1.51
426 Y. Xia et al. / Lithos 184–187 (2014) 416–435
Our geochronological results for six samples from Xiawan andDuntou granites give zircon U–Pb ages of ca. 410 Ma (Fig. 4). Theseages can be interpreted as the crystallization ages of these granites. Itis worth noting that a fraction of zircons reveal comparatively olderages in XW02, DT02 and DT04, and these older zircon grains give aweighted mean 206Pb/238U age of ca. 436 Ma in both XW02 and DT04(Fig. 4). In addition, the Th/U values of such older zircons are magmaticorigin, and the εHf(t) values of older zircons are fully consistent withyounger ones, suggesting that all the 410 Ma and 436 Ma zircons areisogeneic or crystallize from the same parental magma. Many studieshave shown that multiple age populations could exist in one graniteor a single zircon with different age domains. Hence the maximumage difference probably representing the duration of magma cooling-crystallization, may reach more than 50 Ma (Cheng et al., 2007;Gehrels et al., 2009; Lou et al., 2005; Wang et al., 2007a; Yu and Zhao,2007). Thus, there might be a large magma chamber beneath Xiawanand Duntou granites, and the crystallization of zircons began when pa-rental magma was still in the magma chamber. Asthenosphere-derivedmagma (see below) periodically heated the parental magma, andsubsequent cooling events led to the crystallization process of zirconsfrom Xiawan and Duntou granites lasting for 26 Ma from ca. 436 Mato ca. 410 Ma. But de Saint Blanquat et al. (2011) indicates that thelongevity of magma reservoirs may hardly exceed 10 Ma. Thus, Xiawanand Duntou granites may commonly form incrementally without largemagma chamber and the growth of magma bodies is episodic (Annen,2009; Annen et al., 2006; Glazner et al., 2004). Melt is convergedby multiple batches of magma via several extraction events (Brown,2013). Alternatively, these ca. 436 Ma zircons, which are also widely
Cathaysia basement
Yangtze basement
5
0
10
-5
-10
-15
-20500
DM
CHUR
450400350
ε Nd(
t)
Age
a
Fig. 10. (a)Whole-rockNd-isotope compositions of Xiawan and Duntou granites (the area of Yathe Xiawan and Duntou granites. The trend of the terrestrial array is from Vervoort et al. (1999)lower crust from Vervoort et al. (2000). Bulk Silicate Earth (BSE) and Depleted Mantle (DM) frGriffin et al. (2000). Symbols are the same as in Fig. 6. Data sources are the same as inAppendix T2012; Fu et al., 2004; Li, 1991; Li et al., 1989, 2010a, 2010b, 2010c; Peng et al., 2000; Peng et al., 2Zhang, 1993; Xu et al., 2006; Yao et al., 2012; Zhang et al., 2010b, 2010c, 2012; Zhao et al., 201
observed in Mesozoic felsic lavas in SE China (Guo et al., 2012),are inherited from the melting source.
5.2. Melting temperature and pressure conditions
Many reports acknowledged a relationship between zircon crystalli-zation and the interplay of temperature and melt chemistry in control-ling zircon solubility. Watson and Harrison (1983) undertook synthesisexperiments that showed zircon solubility to be a function of thezirconium concentration of the melt, temperature, and magmacomposition. We have applied this relationship to calculate the Zrsaturation temperature of early Paleozoic granites in South China. Theresults show that, in contrast to group B granitoids, which displaycomparatively large temperature range from 609 °C to 884 °C (mostlyon 700–830 °C) (Chen et al., 2012; Cheng et al., 2009a,2009b, 2012;Fu et al., 2004; Geng et al., 2006; He et al., 2010b; Li et al., 2006,2010a,2010c, 2011; Liu et al., 2008; Lou et al., 2002, 2005; Peng et al.,2006a,2006b; Shen et al., 2008; Wan et al., 2010; Wang et al., 2011a,2012b, 2013b; Wu et al., 2008; Xu et al., 2006; Yao et al., 2012; Zenget al., 2008; Zhang et al., 2010b,2010c, 2012; Zhao et al., 2013b; Zhouet al., 1994b), the Group A granitoids, including the Xiawan and Duntougranites, generally record higher temperatures from 751 °C to 853 °C(mostly at 810–850 °C) (Wan et al., 2010; Li et al., 2010b and thisstudy). This is likely due to the fact that the Group A granitoidsform with more significant contributions of hot asthenosphere-derived materials (see below). An contents of plagioclases togetherwith P2O5 and TiO2 saturation thermometer of Xiawan and Duntou
ε Hf(t
)
DM
BSE
MORB
OIB
Global lower crust
Global sediments
Terrestria
l Arra
y
Hf=1.36 Nd
+2.95; =0.86
r2
ε
ε
SCLM?
εNd(t)-10 -5 0 5 10 15
-20
-10
0
10
20
b
ntze and Cathaysia basement is after Chen and Jahn, 1998). (b) εNd(t) vs. εHf(t) diagram for. Fields for MORB, OIB and global sediments are from Vervoort et al. (1999), and for globalom Blichert-Toft and Albarède (1997), a suggested field for the bulk cratonic (SCLM) fromable 1. Data of Sr–Nd isotope are from the following references: Cheng et al., 2009a, 2009b,006a; Shen et al., 2008;Wan et al., 2010;Wang et al., 2011a, 2013b;Wu et al., 2008; Xu and3b and this study.
427Y. Xia et al. / Lithos 184–187 (2014) 416–435
also support the result of zircon saturation temperatures (Fig. 8;Table 2).
As has been noted, the peak metamorphic pressure of the earlyPaleozoic orogen in South China Block was N0.8 GPa, reaching up to1.0–1.1 GPa (Li et al., 2010c; Yu et al., 2003a, 2005a, 2005b, 2007),with the corresponding crustal thickness of 30–40 km. As syn-collisional to post-collisional granites, the stress relaxation processhad begun when early Paleozoic granites started melting and emplace-ment. According to the experimental studies summarized by PatiñoDouce (1999), the compositions of most early Paleozoic granites plotalong the reaction curves that would be produced by the hybridizationof high-A1 olivine tholeiite with metapelite or metagreywacke at lowpressure (≤0.5 Gpa) with the corresponding crustal thickness lessthan ca. 18 km (Fig. 11f), indicating fast crustal thinning after stressrelaxation.
5.3. The geneses of early Paleozoic granites in South China
Based on the studies of Lachlan Fold Belt granitoids, Chappell andWhite (1974) classified them into S- and I-type. As per Chappell et al.(2012), S-type granites always contain more Al than is required toform feldspar, given the Na, K and Ca contents of the rock, and aretherefore peraluminous. The excess Al is hosted in Al-rich biotite,generally accompanied by more Al-rich minerals such as cordierite or
Al2O3/TiO2
1
10
0.1
10.01
100
10
CaO
/Na 2
O
100 1000
40
30
20
10
0
SiO2(wt.%)
c
e
a
40 50 60 70 80
CaO
/(F
eO+
MgO
+T
iO)
T
0 1 2 3 40
5
10
15
20
25
CaO/(MgO+FeOT)
Al 2
O3/
(MgO
+F
eOT)
FeO
T+
MgO
+T
iO2(
wt.%
)
Fig. 11. Source discrimination diagrams for Xiawan and Duntou granites. (a) and (b) are after(f) is after Patiño Douce (1999). Symbols and data sources are the same as in Fig. 7.
muscovite. In contrast, I-type granites are metaluminous and alwayscontain amphibole. Chappell (1984, 1999) argued that S-type granitewas derived by partial melting of supracrustal rocks that haveundergone some extents of weathering. On the other hand, I-typegranite was derived by partial melting of mafic to intermediate meta-igneous crustal rocks that had not experienced surface processes.However, Gray (1984) proposed a two-component mixing model andCollins (1996) argued a three source-component mixing model whichoffers a better understanding of the genesis of granites between I- andS-type with partial characteristics of either I- or S-type and the linearisotopic array or chemical variation of the I–S type granitoids paragenet-ic assemblage. Recently, the integrated in situ U–Pb, Hf, and O isotopestudy of zircons from granites has also demonstrated that I-type granitemay form by mixing of sedimentary materials and mantle-derivedmagmas instead of remelting ancient meta-igneous crustal rocks(He et al., 2010a; Kemp et al., 2007; Yang et al., 2012; Zhu et al.,2009). Clemens et al. (2011) also suspected that the S–I dichotomy ingranite typology is unlikely to reflect simple sedimentary versusigneous sources. But Clemens (2003) argues that the isotopic variationsbetween I- and S-granitoids could be caused by source heterogeneities,magma mixing, assimilation and even by isotopic disequilibrium.If source mixing, magma mixing and wall-rock assimilation do existextensively, the intermediate types between I- and S-granitoids areprobably not minor in volume.
Rb/Sr0.1 1 10 100
0.010.01
0.1
1
10
100
b
d
f
0
0.5
1
1.5
2
2.5
0 5 10 15 20 25 30 35 40 45CaO+FeO+MgO+TiO2(wt.%)
2
0 5 10 15
0.09
0.13
0.17
0.21
0.25
(Na2O+K2O)/(FeO+MgO+TiO2)T
Na 2
O+
K2O
+F
eO+
MgO
+T
iO2
basalt/amphibolites
Rb/
Ba
Lee et al. (2003) and Altherr and Siebel (2002). (c), (d) and (e) are after Sylvester (1998).
428 Y. Xia et al. / Lithos 184–187 (2014) 416–435
According to previous studies, except for a few granites that areconsidered as I-type granites (e.g., Hufang, Sibao, Maixie, Miao'ershan,Yuechengling, Hongjiang, Wugongshan, Shanzhuang, Sheshan andDaning; Chen et al., 2011, 2012; Li et al., 2010b, c; Lou et al., 2005;Nong et al., 2012; Sha and Yuan, 1991; Wu and Zhang, 2003; Zhaoet al., 2013b), most early Paleozoic granites in South China belong toperaluminous S-type (Charvet, 2013). However, although most earlyPaleozoic granites are peraluminous, some of them are metaluminous(Fig. 7b) with relatively low K2O/Na2O values (Fig. 7c) and lack Al-richcordierite or muscovite minerals. Some early Paleozoic granites havehigh P2O5 that decreases with increasing SiO2 (Fig. 8) and show I-typegranites affinity (Chappell, 1999). Thus, in ACF diagram (Fig. 7d) manyearly Paleozoic granites plot in the I-type field or straddle the boundarybetween I- and S-type fields. On the other side, those so- called I-typegranites are always peraluminous with A/CNK = 0.93–1.40 (mosthigher than 1.0). And except for a few granites like Yuechengling,Sheshan and Daning, those so-called I-type granites are generallyamphibole-free. Therefore, the genetic type of early Paleozoic granitesis not clear-cut, and many granites are transitional between S- andI-type, implying that sourcemixing, magmamixing or wall-rock assim-ilation instead of source heterogeneities may give rise to these granites(Clemens, 2003; Clemens et al., 2011). For example, lacking amphibole,Al-rich cordierite, or protogenic muscovite, Xiawan monzograniteand Duntou granodiorite are metaluminous to peraluminous (A/CNK = 0.96–1.48) (Fig. 7b) with variable K2O/Na2O (Fig. 7c) and nega-tive correlation betweenP2O5 and SiO2 (Fig. 8), andmost of themplot inthe S-type field of ACF diagram (Fig. 7d). The compositions of biotites inXiawan and Duntou granites plot in the fields of biotite in peraluminousgranites (S-type granites) and biotite in metaluminous calc-alkaline
Hybridization trend
Hyb
ridiz
atio
n tre
nd
Evolutionary trend Amphibolites
FeO
T
MgO0 4
4
8
12
16
-16
90%
90%
10%
10%10%
30%
DM(DM-derived melt)LFB I-type granites
LFB S-type granitesOrdovician sediments
-12
-8
-480%
0
4
8
ε Nd(
t)
0.7 1 0.7 2 0.7 387Sr/86Sri
90%
10%
30%
70%
50%
47.007.0
80%
30%
70%50%
ε(t
)
-
-
-
crustal assimilationsource contamination
1
0
0.8 12 16 20
a
c
Fig. 12. (a) FeOT vs.MgO diagram (after Zorpi et al., 1991). (b)Distribution of felsic igneous rockvs. initial 87Sr/86Sr and (d) εNd(t) vs. SiO2 diagrams at 410 Ma. The parameters of end-memberet al., 2011a, 2013b; Workman and Hart, 2005 and reference therein). The depleted mantle isεNd(t) = +8.5, and depleted mantle-derived magma by SiO2 = 48 wt.%, Nd = 20 ppm, Srrocks are 09WG-32A1 (SiO2 = 64 wt.%, Nd = 30.2 ppm, Sr = 97.6 ppm, 87Sr/86Sri =Sr = 131.9 ppm, 87Sr/86Sri = 0.71289 and εNd(t) = −9.4), 09WG-32 g (SiO2 = 73.7 wt.%, N(SiO2 = 75.3 wt.%, Nd = 59.3 ppm and εNd(t) = −13.7). Symbols and data sources are the s
granite suites (I-type granites) (Fig. 2c, d). The Xiawan and Duntougranites exhibit high εHf(t) and εNd(t) values (Fig. 6, 10a) and their ini-tial 87Sr/86Sr and εNd(t) are similar to Lachlan Fold Belt I-type granites(Fig. 12c), suggesting that Xiawan andDuntou granities are neither typ-ical I- nor S-type granites but have partial characteristics of either I- orS-type granites. So it seems difficult to accurately classify early Paleozoicgranites in South China following the classification of Lachlan FoldBelt granites into S- and I-type (Chappell and White, 1974). For thepurpose of studying the genesis of early Paleozoic granites, we preferto divide these granites into two Groups A and B using εHf(t) andεNd(t) values.
All the early Paleozoic granites in South China have similar REEand trace elements patterns to the metasedimentary rocks fromPaleoproterozoic to Neoproterozoic metamorphic basement which areinvolved in early Paleozoic Orogen (Liu et al., 2008; Wan et al., 2010;Wang et al., 2011a; Zeng et al., 2008) (Fig. 9). Hf and Nd isotopiccompositions of Group B granitoids are in accordance with thePaleoproterozoic to Neoproterozoic metamorphic basement whileHf and Nd isotopic compositions of Group A are higher (AppendixTable 1; Figs. 6, 10a). In source discrimination diagrams based onexperimental studies of melting of metamorphic rocks, most ofgranitoids from both Group A and Group B plot in the biotite-richmetapelite field and the rest plot into muscovite-rich metapelite field,implying that most of the early Paleozoic granites are derived fromthe dehydration melting of biotite (Fig. 11a, b), which coincide withthe temperature and pressure conditions for graniticmagmageneration(Dini et al., 2005). But the relatively high CaO/Na2O ratios (N0.3) andlow SiO2 contents (part of samples b71 wt.%) imply that these granitescould be produced from psammitic sources or pelite-derived melts that
90% 90%
90%
10%
10%
30%
30%
80%
50%
Nd
15
20
10
-5
0
5
10DM
partial melt orfractionation
crystallization
50 08070604
SiO2
90%
70%
90%
80%
20%
40%30%
80%
70%
Magma-crust
interaction
Deep-crustal
recycling
AFC
Wedgemelting
4%
3%
2%1%
depletedmantle
enrichedmantle
subd
uctio
n
N-MORB
E-MORB
OIB
MORB-OIB
arra
y
volca
nic arc
array
00
10
1
.1
010.1 1 10 100 1000
Nb/Yb
b
d
Th/
Yb
DM-derived melt
s fromdifferent tectonic settings on a Th/Yb–Nb/Yb diagram (after Pearce, 2008). (c) εNd(t)s are selected based on the literature (e.g., Niu and O'Hara, 2003; Wan et al., 2010; Wangrepresented by SiO2 = 45 wt.%, Nd = 4.0 ppm, Sr = 60 ppm, 87Sr/86Sri = 0.70246 and= 400 ppm, 87Sr/86Sri = 0.70246 and εNd(t) = +8.5. Four involved metasedimentary0.73639 and εNd(t) = −10.5), 09WG-32E (SiO2 = 62.5 wt.%, Nd = 19.0 ppm,
d = 29.9 ppm, Sr = 113.3 ppm, 87Sr/86Sri = 0.72617 and εNd(t) = −9.9) and G0105-1ame as in Figs. 7 and 10.
SiO2(wt%)
1
0.1
10
100
7550 55 60 65 70 80
S-COLG
VAG+P-COLG
S-COLG:syn-collisional granitesP-COLG:post-collisional granitesVAG: volcanic arc granitesORG: ocean ridge granites
WPG:within-plate granites
40 45
P-COLG
S-COLG
VAG
WPG
ORG
1 10 100 1000
10
100
1000
Y+Nb(×10-6)
Rb(
×10
-6)
0.088
80%
DM
Bt
KfPl Cpx
Mt
Amp
Compression
Extension
1.0
0
-1.0
-2.0
-3.0
-4.0
65 75 8060])O
aN
+O
K(/O
aC[
gl2
2
SiO 2
Partial meltingdiulf
en
ozn
oitcu
db
us subd
uctio
n zo
ne m
elt
enric
hed
man
tle
a
b
c
Rb/
Zr
70
Fig. 13. Tectonic setting discrimination diagrams: (a) lg[CaO/(K2O + Na2O)] vs. SiO2
discrimination diagram is after Brown (1982); (b) Rb/Zr vs. SiO2 discrimination diagramis after Harris et al. (1986); (c) Rb vs. (Y + Nb) discrimination diagram is after Pearceet al. (1984) and Pearce (1996). Symbols and data sources are the same as in Fig. 7.
429Y. Xia et al. / Lithos 184–187 (2014) 416–435
interacted with mafic magmas (Sylvester, 1998) (Fig. 11c). However,Fig. 11d indicates that quite a number of the early Paleozoic graniteshave Rb/Ba and Rb/Sr ratios lower than psammite-derived melts. TheFeOT, MgO and TiO2 contents of those granites demonstrate that ifthey are derived from partial melting of clay-poor sources, the initialtemperatures must be higher than 900 °C (Fig. 11e), which is highlyunlikely according to the P2O5 and TiO2 saturation thermometer andZr saturation temperature of the early Paleozoic granites.
It'sworth noting that early Paleozoic granites,mafic enclaves (Chenget al., 2009a; Lou et al., 2005; Sha and Yuan, 1991;Wu and Zhang, 2003)and mafic to intermediate rocks (He et al., 2010b; Li et al., 2010a; Penget al., 2006a; Wang et al., 2013b; Yao et al., 2012) or amphibolites (Liet al., 2011) always form a chemical array (Fig. 11c–f). Gray (1984)suggested that mafic enclaves are solidified basaltic dykes, which havebeen physically and chemically modified during ascent and emplace-ment of the granite magma. The FeOT and MgO contents of thesegranites, mafic enclaves and mafic to intermediate rocks lie along
hybridization trend (Fig. 12a), implying some of early Paleozoic granitesin South China form by mixing as previously proposed by Gray (1984).As for Xiawan and Duntou granites, most of their biotites belong tomagnesium biotites (Fig. 2b), which support the input of mantle sourcemagma (Zhou, 1988). Therefore Figs. 11c–f, and 12a suggest themixingof mafic and felsic components as the parent sources of these granites,but the relative proportions of each component for different granitesvary. In addition, the mafic component of mixing model can eitherbe mantle-derived magma or remelting of Paleoproterozoic toNeoproterozoic amphibolites (Li et al., 2011).
Granite generations are usually related with mantle-derivedmagmas underplating, and many researchers (e.g., Patiño Douce,1999; Sylvester, 1998) believed that the interactions between thecontinental crust and underplated mafic magmas range from limitedto mere heat transfer to chemical interactions. Although the earlyPaleozoic granites are most S-type, the peraluminous S-type granitesalso cannot completely rule out the input of mantle-derived magma.For instance, Barbarin (1996, 1999) have demonstrated that theperaluminous CPG-type granitoids generate where mantle-derivedmagma is injected into, or has been underplated. As mentioned above,the peak metamorphism of early Paleozoic orogen occurs earlier than445 Ma and reaches to granulite face with peak metamorphic pressureof 1.0–1.1 GPa and temperatures of 835–878 °C (Yu et al., 2003a,2005a,2005b), followed by a phase of rapid cooling below 500–300 °Cwhich occurs by ca. 420 Ma (Li et al., 2010c). But those post-collisional granites were mainly produced between 440 and 390 Mawith melting temperatures in the range pf 609 °C to 884 °C andpressure ≤ 0.5 Gpa. Clemens (2003) also argued that normal geother-mal gradients and crustal thickening likewise fail to provide sufficientheat for partial melting of the crust. Generally, the mantle must be themajor heat source. Underplating of hot mantle-derivedmagmas heatedand elevated temperature at significantly lower pressure. Thus, theearly Paleozoic S-type granite can form by partial melting of meta-sedimentary rocks from Paleoproterozoic to Neoproterozoic meta-morphic basement upon underplating or injection of mantle derivedmagmas.
Pearce (2008) suggested the Th/Yb–Nb/Yb diagram to highlight thecrustal contamination. In the Th/Yb–Nb/Yb diagram, the variation trendof early Paleozoic granites, mafic enclaves and mafic to intermediaterocks indicates magma–crust interaction (contamination) due to wedgemelting or assimilation fractional crystallization (AFC) (Fig. 12b). Giventhat the early Paleozoic tectothermal event is intracontinental, thegeneration of arc chemical signature may witness a paleo-subductionmodified wedge column preserved from Neoproterozoic subduction(Wang et al., 2013b). The Sr–Nd isotopic array (Fig. 12c) also suggestsa source contamination or crustal assimilation model, and excludesamphibolites as the mafic component. However, because granites can-not be directly derived from the mantle (Wyllie, 1984), contaminationof depleted mantle and metasedimentary rocks from Paleoproterozoicto Neoproterozoic metamorphic basement may not account for thegenesis of the early Paleozoic granites. As per the calculated binarymixing curves using εNd(t) values and SiO2 contents (Fig. 12d), theearly Paleozoic mafic enclaves and mafic rocks form by contaminationof depleted mantle by metasedimentary rocks from Paleoproterozoicto Neoproterozoic metamorphic basement while intermediate rocksand many early Paleozoic granites form by assimilation of mantle-derived magmas and metasedimentary rocks with some level offractionation crystallization. It is hard to image that contamination ofdepleted mantle and metasedimentary rocks give rise the primarymelt of the basaltic magmas, because no subduction took place inearly Paleozoic Cathysia block. The metasomatism must take placemuch earlier, as is the case for Jinan gabbroic intrusion from the NorthChina Block (Guo et al., 2013). Therefore the basaltic magmas werelikely derived from paleo-subduction metasomatized hydrated sub-continental lithosphere mantle (Wang et al., 2013b; Yao et al., 2012),whereas the intermediate to acidic magmas were the products of AFC
430 Y. Xia et al. / Lithos 184–187 (2014) 416–435
processes with interactions between the basaltic magmas and evolvedcrustal materials (Fig. 12b–d). With higher εHf(t) and εNd(t) values(Appendix Table 1; Figs. 6, 10a), the Group A granitoids includingXiawan and Duntou granites may be formed by mixing of moreasthenosphere-derived magma with metasedimentary rocks. Partsof the Group B granitoids, mafic enclaves and mafic to intermediaterocks constitute fine assimilation (mixing of basaltic magma withmetasedimentary rocks) curves, indicating that besides those purelycrust-derived S-type granites, the Group B granitoids may be formedby mixing of synchronous basaltic magma with metamorphic crustalmaterials.
5.4. Implications for geodynamics of early Paleozoic orogenesis anddelamination in South China
In many tectonic settings discrimination diagrams (Brown, 1982;Harris et al., 1986; Pearce, 1996; Pearce et al., 1984), the early Paleozoicgranites in South China show characteristics of granitoids producedin extensional environment (Fig. 13a), and most plot in the post-collisional and volcanic arc granites fields while the rest plot in thesyn-collisional fields (Figs. 7b, 13b, c). As show in Fig. 13c, some early
Changsha
W
Yangtze
Paleo-subduction metasSub-continental lithosph
a >460 Ma Anhua-Luocheng Fau
Chenzhou-Linwu Fault
Changsha
Tanghu
Ecologiticlower crust
300 Co
700 Co
b 460-445 Ma
A
300 Co
700 Co
1300 Co Asthenosphericupwelling
c 440-390 Ma
Changsha
TanghuPenggongmiao Epo Ningh
Miao’ershanYuechengling Wanyangshan
Hongxiaqiao Banshanpu
Fig. 14. Schematic cartoons showing the petrogenetic mechanism for early Paleozoic grani(modified after Li et al., 2010c; Yao et al., 2012; Wang et al., 2013b).
Paleozoic granites including Xiawan and Duntou granites plot inthe volcanic arc granites fields, as a result of fractional crystallizationof K-feldspar and plagioclase, mixing of mantle-derived magmas orpartialmelting of subductionmodified rocks. However, because Xiawanand Duntou granites show weak positive or negative Eu anomalies(Eu/Eu* = 0.42–1.33) and negative Sr anomalies, the fractional crystal-lization of K-feldspar and plagioclase must be insignificant (Table 3,Fig. 9). In fact, these tectonic setting discrimination diagrams havetheir own limitations, as pointed out by Förster et al. (1997) andPearce et al. (1984), and granite compositions essentially depend onthe nature of the source rocks, and are controlled only in the simplestcases by the tectonic setting. Thus as has been noted, the arc-likecharacter may be derived from the remelting of paleo-subductionmodified wedge column preserved from Neoproterozoic subduction(Wang et al., 2013b). Such a character was inherited by basalticmagmas that injected into, or underplated beneath, the crust andinduced its melting to form the parental magma of early Paleozoicgranites. Therefore, based on the geochronological studies and theformation of synchronous mafic rocks (Wang et al., 2013b; Yao et al.,2012), most of early Paleozoic granites belong to post-collisionalgranites and form in an extensional environment.
Nanchang
uyi Domain
Catha siay
Pingxiang-Jiangshan-Shaoxing Fault
Crust
omatized hydratederic mantle (SCLM)
Astheosphere
lt
Nanchang
Ninghua
B
C
Asthenosphericupwelling
Delamination
Delamination
NanchangXiawanDuntou
ShaxiWeipuua
tes in South China as projected on cross section A–B and vertical section B–C in Fig. 1
431Y. Xia et al. / Lithos 184–187 (2014) 416–435
The typical model of an orogenic cycle generally begins with crustalshortening leading to crust thickening, prograde metamorphism andhigh topography at the orogenic core. Subsequently the thickenedcrust resists further shortening and crustal root often experiencesgranulite or eclogite facies metamorphism. If the crustal root undergoesmineral equilibration under eclogite facies and becomes denser than theunderlying mantle, it usually causes the delamination of both eclogiticlower crust and the attached lithospheric mantle (e.g., Kay and Kay,1993). Finally, accompanying the decompression and retrogrademetamorphism, late-orogenic tectonic collapse results from mantle–crust isostatic re-equilibrium (Arndt and Goldstein, 1989; Froidevauxand Ricard, 1987; Leech, 2001; Schott and Schmeling, 1998).
As discussed above, early Paleozoic granites form by partial meltingof metasedimentary rocks from Paleoproterozoic to Neoproterozoicmetamorphic basement induced by underplating or injection of hotmantle-derived magmas. The temperature and pressure evolutionprocess of early Paleozoic orogenesis and magmatism also indicatefast crustal thinning and confirm that underplating of hot mantle-derived magmas heated and elevated temperature to induce theorogenic root melting and crust–mantle interaction. Nevertheless,lithosphere extension and decompression in the late-orogenic stageare not enough to trigger the subcontinental mantle melting, even ifhydrated. This is because, at pressures less than about 2 GPa, ascentpaths in the subcontinental mantle are nearly parallel to the basaltsolidus, and so little melting occurs if the lithosphere is thinner thanabout 60 km (Harry and Leeman, 1995). Yao et al. (2012) suggestedthat the lower crust might undergo eclogite facies metamorphism. TheHf–Nd isotopic studies indicate the lower crust origin for Xiawan andDuntou granites (Fig. 10b). Some samples from these two granitesexhibit weak positive Eu anomalies, implying that they may originfrom mafic granulitic or eclogitic lower crust, yet portions with weaknegative Eu anomalies may derive from the crust after mafic granuliticor eclogitic lower crust has delaminated (Gao et al., 1992, 1999a,1999b, 2004; Gao and Wedepohl, 1995; Rudnick, 1995; Vanderhaegheand Teyssier, 2001; Wedepohl, 1991) (Table 3, Fig. 9). Geochemicaland isotopic analyses on the mafic enclaves and basic rocks indicatethat the initial basaltic magmas were likely derived from apaleosubduction metasomatized hydrated sub-continental lithospheremantle (SCLM) (Wang et al., 2013b; Yao et al., 2012) (Fig. 12b–d), andthe melting temperature for the basaltic magmas and the calculatedmantle potential temperature are both ca. 1300 °C, similar to that of aMORB-like asthenospheric mantle. This supports the hypothesis thatthe partially molten SCLM was heated up by upwelling asthenospheretriggered by the removal of the delaminated lithosphere (Yao et al.,2012). Yao et al. (2012) also predicted asthenospheric melts due to de-compression after the delamination and suggested that suchmelts like-ly contributed to the basaltic underplating beneath the orogen.However, outcrop of such magmatism had not been identified. TheGroup A granitoids including Xiawan and Duntou granites form byAFC processes with interactions between asthenosphere-derivedmagma and metasedimentary rocks of metamorphic basement, whichmay serve as the evidence for the direct participation of asthenosphere.
Based on previous studies and this work, we try to elucidate thesequence of processes that occurred during the early Paleozoic in SouthChina and improve the understanding of synchronous magmatisms:
(1) The early Paleozoic Orogen in South China Block began at no laterthan 460 Ma (Middle Ordovician) (Li et al., 2010c) involvedwithpaleo-subduction metasomatized hydrated SCLM (Wang et al.,2013b; Yao et al., 2012) and metasedimentary rocks fromPaleoproterozoic to Neoproterozoic metamorphic basement(Fig. 14a).
(2) At 460 to 445 Ma (Fig. 14b), with crustal shortening leadingto crust thickening, prograde metamorphism attained the peakmetamorphic pressure of 1.0–1.1 GPa and temperatures of835–878 °C with a series of thrust faults developed (Li et al.,
2010c; Yu et al., 2003a, 2005a, 2005b, 2007), and a fraction ofthe metasedimentary rocks turned to melt and syn-collisionalgranites occurred (e.g., Tanghu, Ninghua and Hongjiang,Wu and Zhang, 2003; Zhang et al., 2010a, 2012). After peakmetamorphism, metamorphic rocks rapid cooled down below500–300 °C at ca. 420 Ma (Li et al., 2010c), owing to exhumationwhich caused by gravitational instability and collapsing orthermal relaxation. Anatexis of thickened crust is no longer themain way of granitic magmatism.
(3) From 440 to 390 Ma (Fig. 14c), as the result of crust thickening,the crustal root underwent mineral equilibration under eclogitefacies, causing the delamination of both the eclogitic lowercrustal and the attached lithospheric mantle with rapidunroofing and collapsing. The delaminated eclogitic lower crust,along with the attached SCLM, dropped into the hotter astheno-sphere, promoting regional asthenospheric upwelling and thepartial melting of paleo-subduction metasomatized hydratedSCLM (Li et al., 2010c; Yao et al., 2012; Wang et al., 2013b andthis study). Due to the partial melting of the SCLM, basalticmagmas generated and underplated, then induced the partialmelting of metasedimentary rocks and the AFC processeswith interactions between the basaltic magmas and crustalmaterials to produce both pure crust-derived granitic magmas(e.g., Penggongmiao, Wanyangshan and Epo, Chu et al., 2012;Wu et al., 2008; Zhang et al., 2012) and crust–mantle hybridizedgranitic magmas (e.g., Miao'ershan, Yuechengling, Banshanpuand Hongxiaqiao, Chu et al., 2012; Xu and Zhang, 1993; Xu andZhang, 1993Xu et al., 2006; Yang et al., 2012; Zhang et al.,2012; Zhao et al., 2013b) of Group B. Meanwhile, the AFC pro-cesses with interactions between asthenosphere-derivedmagma and metasedimentary rocks of metamorphic basementproduced granitic magmas of Group A (e.g., Weipu, shaxi,Xiawan and Duntou Li et al., 2010b and this study).
6. Conclusions
(1) This study yields the crystallization age of Xiawanmonzograniteand Duntou granodiorite of ca. 410 Ma. The older group ages ofca. 436 Ma found in both Xiawan and Duntou granites imply anincremental construction or episodic growth model or theseolder zircons are inherited from the melting source.
(2) The early Paleozoic granites in South China generated mainlyduring 440 and 390 Ma are mostly post-collisional granites.With the exception of some typical S-type granites or I-typegranites, many granites show petrological and geochemicalcharacteristics of either I- or S-type. To avoid the ambiguousclassification of I–S type, we prefer to divide all early Paleozoicgranites into two groups: Group A with relatively high εHf(t)and εNd(t) values together with high initial temperatures, andGroup B with relatively low εHf(t) and εNd(t) values togetherwith low initial temperatures.
(3) Geochemical and isotopic analyses on early Paleozoic granitesand relative rocks demonstrate that the Group A granitoidsincluding Xiawan and Duntou granites may form by AFCprocesses with interactions between asthenosphere-derivedmagma and metasedimentary rocks. Except for those purecrust-derived S-type granites, Group B granitoids may form byAFC processes with interactions between synchronous basalticmagma and metamorphic basement.
(4) The Hf–Nd isotopic characteristics and weak positive ornegative Eu anomalies in Xiawan and Duntou granitesimply a delaminated eclogitic lower crust. Our petrogeneticstudies of Group A granitoids also suggest the underplating ofasthenospheric melts and support the delamination model. Thedelamination of eclogitic lower crust along with the attachedSCLM at post-collisional stage triggered regional upwelling, and
432 Y. Xia et al. / Lithos 184–187 (2014) 416–435
led to partial melting of asthenosphere and of the paleo-subduction metasomatized hydrated SCLM, inducing thegenerations of Group A and Group B granitoids, respectively.
Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.lithos.2013.11.014.
Acknowledgments
This work was supported by National Basic Research Programof China (2012CB416701) and National Natural Science Foundationof China (Grant 41072043) and a Specialized Research Fund for theDoctoral Program of Higher Education (20120091110022). We arethankful to Chen, F. K. for his assistance with Sr–Nd isotope analysis,and to Tang, H. F. for his assistance with zircon Hf isotope analysis.
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